Preparation of Nanostructured Sn/Ti Oxide Hybrid Films with Terpineol/PEG-Based Nanofluids: Perovskite Solar Cell Applications

Tin oxide (SnO2) and titanium dioxide (TiO2) are recognized as attractive energy materials applicable for lead halide perovskite solar cells (PSCs). Sintering is one of the effective strategies for improving the carrier transport of semiconductor nanomaterials. Using the alternative metal-oxide-based ETL, nanoparticles are often used in a way that they are dispersed in a precursor liquid prior to their thin-film deposition. Currently, the creation of PSCs using nanostructured Sn/Ti oxide thin-film ETL is one of the topical issues for the development of high-efficiency PSCs. Here, we demonstrate the preparation of terpineol/PEG-based fluid containing both tin and titanium compounds that can be utilized for the formation of a hybrid Sn/Ti oxide ETL on a conductive substrate (F-doped SnO2 glass substrate: FTO). We also pay attention to the structural analysis of the Sn/Ti metal oxide formation at the nanoscale using a high-resolution transmission electron microscope (HR-TEM). The variation of the nanofluid composition, i.e., the concentration of tin and titanium sources, was examined to obtain a uniform transparent thin film by spin-coating and sintering processes. The maximum power conversion efficiency was obtained for the concentration condition of [SnCl2·2H2O]/[titanium tetraisopropoxide (TTIP)] = 25:75 in the terpineol/PEG-based precursor solution. Our method for preparing the ETL nanomaterials provides useful guidance for the creation of high-performance PSCs using the sintering method.


Introduction
Thin solid films consisting of semiconductor nanomaterials have received increasing attention for their potential applications, such as optoelectronic devices [1][2][3]. Among them, perovskite solar cells (PSCs) built from multi-semiconductor layers including halide perovskite light absorbers are now attracting widespread interest, in view of not only the intriguing basic science of related materials but also their commercialization [4][5][6]. These devices are also expected to be integrated into versatile renewable energy systems [7]. In particular, with the need for alternative energy sources, materials research for PSC applications has gained tremendous progress in the improvement of the light-to-electricity conversion of PSCs, as well as the durability and diversity of devices with various transparent conductive substrates. One of the major device structures of PSCs realizing a decent power conversion efficiency includes the use of metal oxide nanoparticle layers that are deposited on top of conductive F-doped tin oxide (FTO) glass substrates [4]. These are known as mesoscopic-type or planar-type devices, for which lead halide perovskite is crystallized adjacent to the metal oxide layers. As for the bottom layer, the following two processes are alternatively employed: annealing-free deposition at temperatures as metal oxide samples, which were prepared by mixing Tin(II) chloride and Titanium(IV) isopropoxide in a suitable base liquid and additive. Notably, we found out that nanofluid preparation using a terpineol/polyethylene glycol (PEG) medium is a key process to obtaining a uniform thin film. In addition, the formation mechanism of Sn/Ti oxide nanoparticles involving the elimination of the base liquid and additive was clarified. The sintering effects of nanostructured semiconductor layers on device performance were discussed towards the development of lead halide PSCs showing high efficiencies.

Preparation of Sn/Ti Oxide Thin Films
F-doped tin oxide glass substrates (11 Ω/ Nippon Sheet Glass, Tokyo, Japan) were washed and cleaned with UV-ozone treatment. We first mixed 40 wt% (0.7 g) of ethanol with 40 wt% (0.7 g) of terpineol as a base liquid and then added 20 wt% (0.35 g) of PEG400 to prepare a base solvent.
Several precursor solutions changing the concentrations of metal sources were prepared as follows: To the mixture of base liquid and PEG, SnCl 2 ·2H 2 O and TTIP were added. The ratios of the metal sources were [SnCl 2 ]/[TTIP] = 100:0, 75:25, 50:50, 25:75, 0:100 (mol/mol). The concentrations of the metal sources in all those solutions were 10 wt%. For example, 0.39 g of SnCl 2 ·2H 2 O was used for preparing the 1:0 condition. Typically, the 10 wt% solution containing metal sources was stirred for 30 min at ambient temperature, followed by spin-coating on top of the FTO substrate at 3000 rpm for 30 s. The substrates were immediately heated on a hot plate at 120 • C for 20 min.

Sintering Process
The substrates were placed in an electric furnace, KDF 300 Plus (DENKEN-HIGHDENTAL, Kyoto, Japan) for the sintering process. In regard to the sintering strategy, the temperature was increased from room temperature to 500 • C. The two different ramping times of 2 h and 30 min were examined. After the maximum temperature was kept at 500 • C, it was then cooled down naturally. The periods of maximum temperature were 2 h and 30 min, respectively (see Figure 1).

Figure 1.
Sintering strategies of the Sn/Ti oxide thin-film samples in air. The periods of maximum temperature at 500 °C were 2 h for (a) and 30 min for (b), respectively. From 500 °C, the samples were cooled down to room temperature naturally.

Evaluation of Sn/Ti Oxide Thin Films
Nanostructures of TiO2 films after nanofluid pool boiling were characterized by scanning electron microscopy (SEM) of HITACHI S-4800 (Hitachi, Ltd., Tokyo, Japan). The transmittance spectrum of the Sn/Ti oxide film was measured using PerkinElmer Lambda950 (PerkinElmer, Waltham, MA, USA). The perovskite solar cell was assembled using the modified method using a triple-cation type lead halide perovskite that has been demonstrated previously [14].
After the sintering process of ETL, UV-ozone treatment was conducted prior to spincoating the halide perovskite precursor solution. Nitrogen was flowed into a glove box to keep the humidity at around 20 %. The substrates were placed in the glove box and heated to 80 °C on a hot plate. To prepare a perovskite precursor solution with a composition of Cs0.05MA0.1FA0.85PbI2.9Br0.1·0.05PbI2, MABr 0.14 mmol (0.0157 g), CsI 0.07 mmol (0.0182 g), FAI 1.19 mmol (0.204 g), and PbI2 1.45 mmol (0.666 g) were dissolved in the mixed solvent of DMF 80 vol% (0.8 mL) and DMSO 20 vol% (0.2 mL). It was then stirred on a hot plate in the dark at 80 °C for 2 h. The heated substrates at 80 °C were set on the spin-coater, and 100 µL of perovskite precursor solution was dropped onto the substrate. A two-step program of spin-coater (1000 rpm for 10 s and 6000 rpm for 30 s) was used. A 200 µL volume of CB was dropped with an electronic micropipette 25 s after the start of spincoating. The substrate was annealed using a hot plate at 100 °C for 1 h, and it was allowed to cool naturally to room temperature. Then, 0.12 mmol (0.0148 g) of FABr was added to 4 mL of IPA to prepare a solution (30 mM), and it was spin-coated on top of the crystallized perovskite layer. A 100 µL volume of the prepared FABr solution was coated at 3000 rpm for 30 s. The substrate was heated on a hot plate at 80 °C for 10 min. After heating, the substrate was allowed to cool to room temperature.
To prepare a solution for the hole transporting layer (HTL), 0.037 mmol (0.045 g) of Spiro-MeOTAD was dissolved in 0.5 mL of CB. Next, 0.18 mmol (0.052 g) of Li-TFSI was dissolved in 0.1 mL of acetonitrile. A 10 µL volume of the acetonitrile solution and 17.75 µL of TBP were added to the solution containing Spiro-MeOTAD. The substrate was cooled to room temperature and spin-coated at 3000 rpm for 30 s. After 5 s from the start of spin-coating, 60 µL of the solution was dropped. The substrates were left for 24 h. Finally, a gold layer was deposited on top of Spiro-MeOTAD by using the thermal evaporation method [15].

Evaluation of Sn/Ti Oxide Thin Films
Nanostructures of TiO 2 films after nanofluid pool boiling were characterized by scanning electron microscopy (SEM) of HITACHI S-4800 (Hitachi, Ltd., Tokyo, Japan). The transmittance spectrum of the Sn/Ti oxide film was measured using PerkinElmer Lambda950 (PerkinElmer, Waltham, MA, USA). The perovskite solar cell was assembled using the modified method using a triple-cation type lead halide perovskite that has been demonstrated previously [14].
After the sintering process of ETL, UV-ozone treatment was conducted prior to spincoating the halide perovskite precursor solution. Nitrogen was flowed into a glove box to keep the humidity at around 20 %. The substrates were placed in the glove box and heated to 80 • C on a hot plate. To prepare a perovskite precursor solution with a composition of Cs 0.05 MA 0.1 FA 0.85 PbI 2.9 Br 0.1 ·0.05PbI 2 , MABr 0.14 mmol (0.0157 g), CsI 0.07 mmol (0.0182 g), FAI 1.19 mmol (0.204 g), and PbI 2 1.45 mmol (0.666 g) were dissolved in the mixed solvent of DMF 80 vol% (0.8 mL) and DMSO 20 vol% (0.2 mL). It was then stirred on a hot plate in the dark at 80 • C for 2 h. The heated substrates at 80 • C were set on the spin-coater, and 100 µL of perovskite precursor solution was dropped onto the substrate. A two-step program of spin-coater (1000 rpm for 10 s and 6000 rpm for 30 s) was used. A 200 µL volume of CB was dropped with an electronic micropipette 25 s after the start of spin-coating. The substrate was annealed using a hot plate at 100 • C for 1 h, and it was allowed to cool naturally to room temperature. Then, 0.12 mmol (0.0148 g) of FABr was added to 4 mL of IPA to prepare a solution (30 mM), and it was spin-coated on top of the crystallized perovskite layer. A 100 µL volume of the prepared FABr solution was coated at 3000 rpm for 30 s. The substrate was heated on a hot plate at 80 • C for 10 min. After heating, the substrate was allowed to cool to room temperature.
To prepare a solution for the hole transporting layer (HTL), 0.037 mmol (0.045 g) of Spiro-MeOTAD was dissolved in 0.5 mL of CB. Next, 0.18 mmol (0.052 g) of Li-TFSI was dissolved in 0.1 mL of acetonitrile. A 10 µL volume of the acetonitrile solution and 17.75 µL of TBP were added to the solution containing Spiro-MeOTAD. The substrate was cooled to room temperature and spin-coated at 3000 rpm for 30 s. After 5 s from the start of spin-coating, 60 µL of the solution was dropped. The substrates were left for 24 h. Finally, a gold layer was deposited on top of Spiro-MeOTAD by using the thermal evaporation method [15].
Current-voltage curves were obtained under AM 1.5 simulated sunlight (100 mW/cm 2 ) using a solar simulator (Yamashita Denso, YSS-80A, Tokyo, Japan) and potentiostat (Hokuto Denko HSV-110, Tokyo, Japan). The active area of the device was 0.09 cm 2 . The currentvoltage curves were measured under reverse scanning conditions at a scan rate of 50 mV/s.

Results and Discussions
The fluid containing precursor materials of tin and titanium ions plays a vital role in determining the resultant film characteristics, such as uniformity of the obtained nanoparticle layers and density of nanocrystals in the film. We attempted to establish a sintering protocol of Sn/Ti oxide thin films applicable for electron transport layers in PSCs. In principle, base liquids and/or additive compounds in the fluid are required to optimize both the concentration of the metal ion sources and the packing structure of formed nanoparticles during heat treatment. By increasing the temperature, hydrolysis and polycondensation of those metal ions start to promote the oxide nanoparticle formation, where the homogeneous reactions should be adjusted in the precursor film. In addition to the use of both ethanol and terpineol to dissolve metal ions, we added a PEG polymer in order to disperse the obtained nanoparticles in the film. PEG is well-known to stabilize metal oxide nanoparticles preventing agglomeration, such as TiO 2 . The sizes of those nanoparticles can be reduced by steric repulsion.
The preliminary experiments allowed us to use the mixed base liquid, i.e., ethanol/terpineol, because the resultant fluid using only ethanol did not give a uniform film after the sintering treatment and instead some cloudy spots were observed in the film. We anticipated that terpineol would be a key chemical, as has been disclosed in the nanoparticle sintering research on dye-sensitized solar cells [16]. In this device, TiO 2 nanoparticle films, which is the host material of light-absorbing dye molecules, can be prepared by a similar sintering approach with a precursor solution comprising suitable base liquids and additives.
The concentration dependence of tin and titanium ions on the film quality was systematically examined, and typical photos of the processed films after sintering are depicted in Figure 2. White agglomerated particles formed when terpineol was not used as a base liquid (Figure 2d-f), whereas transparent compact thin films with an approximate thickness of 100 nm as mentioned later were obtained for the ethanol/terpineol base liquid (Figure 2a-c). It appears that the terpineol with the higher boiling point (~100 • C) than ethanol assisted the homogenous reaction as a solvent, leading to the formation of welldispersed nanoparticle films. It is likely that PEG also worked as a base liquid similar to the other solvents in a way that the inhomogeneous reactions were retarded due to the higher boiling temperature (>200 • C) than those of ethanol and terpineol. 0.09 cm 2 . The current-voltage curves were measured under reverse scanning conditions at a scan rate of 50 mV/s.

Results and Discussions
The fluid containing precursor materials of tin and titanium ions plays a vital role in determining the resultant film characteristics, such as uniformity of the obtained nanoparticle layers and density of nanocrystals in the film. We attempted to establish a sintering protocol of Sn/Ti oxide thin films applicable for electron transport layers in PSCs. In principle, base liquids and/or additive compounds in the fluid are required to optimize both the concentration of the metal ion sources and the packing structure of formed nanoparticles during heat treatment. By increasing the temperature, hydrolysis and polycondensation of those metal ions start to promote the oxide nanoparticle formation, where the homogeneous reactions should be adjusted in the precursor film. In addition to the use of both ethanol and terpineol to dissolve metal ions, we added a PEG polymer in order to disperse the obtained nanoparticles in the film. PEG is well-known to stabilize metal oxide nanoparticles preventing agglomeration, such as TiO2. The sizes of those nanoparticles can be reduced by steric repulsion.
The preliminary experiments allowed us to use the mixed base liquid, i.e., ethanol/terpineol, because the resultant fluid using only ethanol did not give a uniform film after the sintering treatment and instead some cloudy spots were observed in the film. We anticipated that terpineol would be a key chemical, as has been disclosed in the nanoparticle sintering research on dye-sensitized solar cells [16]. In this device, TiO2 nanoparticle films, which is the host material of light-absorbing dye molecules, can be prepared by a similar sintering approach with a precursor solution comprising suitable base liquids and additives.
The concentration dependence of tin and titanium ions on the film quality was systematically examined, and typical photos of the processed films after sintering are depicted in Figure 2. White agglomerated particles formed when terpineol was not used as a base liquid (Figure 2d-f), whereas transparent compact thin films with an approximate thickness of 100 nm as mentioned later were obtained for the ethanol/terpineol base liquid (Figure 2a-c). It appears that the terpineol with the higher boiling point (~100 °C) than ethanol assisted the homogenous reaction as a solvent, leading to the formation of well-dispersed nanoparticle films. It is likely that PEG also worked as a base liquid similar to the other solvents in a way that the inhomogeneous reactions were retarded due to the higher boiling temperature (>200 °C) than those of ethanol and terpineol. Samples (a-c) were prepared by using a precursor solution of tin and titanium compounds dissolved in a mixed ethanol/terpineol base liquid. The ratios of tin and titanium ions (Sn/Ti) in the precursor were 25:75 for (a), 50:50 for (b), and 100:0 for (c), respectively. Samples (d-f) were prepared via the same sintering strategy as those of (a-c). An ethanol base liquid without terpineol was employed for the preparation of samples (d-f). Similarly, the ratios of Sn/Ti in the precursor were 25:75 for (d), 50:50 for (e), and 100:0 for (f), respectively. For samples (a-c), the right parts highlighted in a dotted yellow line were not covered with Sn/Ti oxide thin films for fabricating perovskite solar cells. The three samples (d-f) showed the formation of agglomerated Sn/Ti oxides on top of the FTO substrates, whereas uniform films formed for (a-c) through the optimization of the composition of precursor solutions.
X-ray diffraction patterns (XRD) of the Sn/Ti oxide samples sintered at 500 • C were analyzed to structurally identify crystalline compounds. Figure 1 depicts the two sintering strategies changing the ramping time, (a) 2 h and (b) 30 min, and periods of maximum temperature, (a) 2 h and (b) 30 min. As a result of heat treatment, XRD patterns exhibiting several broad peaks were obtained, which are presented in Figure 3. X-ray diffraction patterns (XRD) of the Sn/Ti oxide samples sintered at 500 °C were analyzed to structurally identify crystalline compounds. Figure 1 depicts the two sintering strategies changing the ramping time, (a) 2 h and (b) 30 min, and periods of maximum temperature, (a) 2 h and (b) 30 min. As a result of heat treatment, XRD patterns exhibiting several broad peaks were obtained, which are presented in Figure 3.
For 30 min sintering samples, the estimated crystallite sizes were 19 nm, 3 nm, and 6 nm for 100:0, 75:25, and 50:50, respectively. For 2 h sintering, the crystallite sizes were 4 nm, 3 nm, and 3 nm for 100:0, 75:25, and 50:50, respectively. Additionally, it turned out that TiO2 was not well crystallized for all those conditions. The more efficient crystallization at lower temperatures for SnO2 probably suppressed the TiO2 For 30 min sintering samples, the estimated crystallite sizes were 19 nm, 3 nm, and 6 nm for 100:0, 75:25, and 50:50, respectively. For 2 h sintering, the crystallite sizes were 4 nm, 3 nm, and 3 nm for 100:0, 75:25, and 50:50, respectively. Additionally, it turned out that TiO 2 was not well crystallized for all those conditions. The more efficient crystallization at lower temperatures for SnO 2 probably suppressed the TiO 2 crystallization during the heat treatment. The time-dependent analysis via heat treatment suggested that crystal growth was not enhanced with the prolonged heating.
In contrast, TiO 2 crystallization was observed both for Sn/Ti = 25:75 and 0:100. For the smaller amount of Sn source (Sn/Ti = 25:75), it is likely that one of the components of the base liquid, i.e., PEG, retards the crystal growth of SnO 2 , allowing to produce the Rutile TiO 2 with estimated crystal sizes of less than 2 nm. Since SnO 2 has a tendency to form a rutile crystal face, this may lead to the Rutile TiO 2 formation, whereas for 0:100, only Anatase TiO 2 grew as a result of the heat treatment.
To investigate the mechanism of film formation of Sn/Ti oxide hybrid films, a transmission electron microscope (TEM) was utilized in order to clarify the microstructure of nanoparticles at a single nanometer scale associated with the formation of mixed oxide layers from titanium and tin compounds. Figures 4 and 5 present several TEM images obtained with different resolutions (Sn/Ti = 50:50), where the images from different positions are also depicted to show the observed uniformity of the samples. The samples prepared by two different sintering strategies (as shown in Figure 1) were measured, and the results are separately illustrated in Figures 4 and 5. As can be seen in Figure 4a,b, it turned out that single-nanometer-scale particles with the approximate dimension of 3~5 nm were formed during heat treatment. The HR-TEM images in Figure 4c indicate lattice fringes exhibiting the (110) and (101) planes of rutile crystal-phase SnO 2 and the (110) plane of rutile TiO 2 . These d-spacings were 0.33 nm, 0.26 nm, and 0.31 nm, respectively. In fact, the calculated crystallite size was consistent with that of the primary nanocrystal size observed in the images that were measured at different positions ( Figure 4d). Irregular-shaped nanocrystals as a result of their fusion were more striking for the longer sintering (the period of maximum temperature: 2 h) than those of 30 min sintering (Figure 5c,d). In other words, more densely packed films were obtained for the 2 h sintering. Similarly, it was found from HR-TEM images of Figure 5c that lattice fringes were assigned to the (110) of rutile SnO 2 and the (101) plane of rutile TiO 2 . These d-spacings were 0.33 nm and 0.25 nm, respectively.
Materials 2023, 16, x FOR PEER REVIEW 5 of 5 crystallization during the heat treatment. The time-dependent analysis via heat treatment suggested that crystal growth was not enhanced with the prolonged heating. In contrast, TiO2 crystallization was observed both for Sn/Ti = 25:75 and 0:100. For the smaller amount of Sn source (Sn/Ti = 25:75), it is likely that one of the components of the base liquid, i.e., PEG, retards the crystal growth of SnO2, allowing to produce the Rutile TiO2 with estimated crystal sizes of less than 2 nm. Since SnO2 has a tendency to form a rutile crystal face, this may lead to the Rutile TiO2 formation, whereas for 0:100, only Anatase TiO2 grew as a result of the heat treatment.
To investigate the mechanism of film formation of Sn/Ti oxide hybrid films, a transmission electron microscope (TEM) was utilized in order to clarify the microstructure of nanoparticles at a single nanometer scale associated with the formation of mixed oxide layers from titanium and tin compounds.  Figure 1) were measured, and the results are separately illustrated in Figures 4 and 5. As can be seen in Figure 4a,b, it turned out that single-nanometer-scale particles with the approximate dimension of 3~5 nm were formed during heat treatment. The HR-TEM images in Figure 4c indicate lattice fringes exhibiting the (110) and (101) planes of rutile crystal-phase SnO2 and the (110) plane of rutile TiO2. These d-spacings were 0.33 nm, 0.26 nm, and 0.31 nm, respectively. In fact, the calculated crystallite size was consistent with that of the primary nanocrystal size observed in the images that were measured at different positions (Figure 4d). Irregular-shaped nanocrystals as a result of their fusion were more striking for the longer sintering (the period of maximum temperature: 2 h) than those of 30 min sintering ( Figure  5c,d). In other words, more densely packed films were obtained for the 2 h sintering. Similarly, it was found from HR-TEM images of Figure 5c that lattice fringes were assigned to the (110) of rutile SnO2 and the (101) plane of rutile TiO2. These d-spacings were 0.33 nm and 0.25 nm, respectively.    In contrast, in the Sn/Ti oxide hybrid film prepared with Sn/Ti = 25:75 condition (Figure 6), the TEM measurement showed very weak crystallinity. This made it difficult to estimate the primary particle sizes because of the aggregation of the particles. In contrast, in the Sn/Ti oxide hybrid film prepared with Sn/Ti = 25:75 condition (Figure 6), the TEM measurement showed very weak crystallinity. This made it difficult to estimate the primary particle sizes because of the aggregation of the particles. Materials 2023, 16, x FOR PEER REVIEW 5 of 5 Figure 6. A TEM image of the Sn/Ti oxide sample treated at 500 °C in air (the ratio of Sn/Ti in the precursor solution was 25:75). The period of maximum temperature in the sintering was 30 min. Figure 7 illustrates a device structure of PSCs employing Sn/Ti oxide films as the electron transport layer (ETL) evaluated in this work. In principle, the light absorption of the perovskite layer sandwiched between an ETL and the hole transporting layer (HTL) enables charge separation in PSCs. The photocarriers (e − and h + ) can be moved to each electrode to achieve photoenergy conversion. Here, our focus was centered on the ratio of Sn/Ti in the precursor solutions of terpineol/PEG base liquids that can optimize the lightto-electricity conversion efficiency of the device.  Figure 8 shows the I-V curve of the best-performing cell measured under one sunlight irradiation (100 mW/cm 2 ). Additionally, Table 1 summarizes the I-V parameters. A maximum power conversion efficiency (PCE) of 18.1% was obtained for the solar cell fabricated using a sintering strategy (ramping time: 30 min, period of maximum   Figure 7 illustrates a device structure of PSCs employing Sn/Ti oxide films as the electron transport layer (ETL) evaluated in this work. In principle, the light absorption of the perovskite layer sandwiched between an ETL and the hole transporting layer (HTL) enables charge separation in PSCs. The photocarriers (e − and h + ) can be moved to each electrode to achieve photoenergy conversion. Here, our focus was centered on the ratio of Sn/Ti in the precursor solutions of terpineol/PEG base liquids that can optimize the lightto-electricity conversion efficiency of the device.  Figure 8 shows the I-V curve of the best-performing cell measured under one sunlight irradiation (100 mW/cm 2 ). Additionally, Table 1 summarizes the I-V parameters. A maximum power conversion efficiency (PCE) of 18.1% was obtained for the solar cell fabricated using a sintering strategy (ramping time: 30 min, period of maximum  Figure 8 shows the I-V curve of the best-performing cell measured under one sunlight irradiation (100 mW/cm 2 ). Additionally, Table 1 summarizes the I-V parameters. A maximum power conversion efficiency (PCE) of 18.1% was obtained for the solar cell fabricated using a sintering strategy (ramping time: 30 min, period of maximum temperature: 30 min) when the Sn/Ti was 25:75. Figure 9 presents the distributions of I-V parameters obtained from each solar cell consisting of the 10 different ETLs. The data clearly show that there is a correlation between J sc and PCE. In other words, the enhanced J sc led to a marked improvement in PCEs. Considering that the samples (Sn/Ti = 100:0, 75:25, 50:50) had higher crystallinity compared to those of Sn/Ti = 25:75, it is most likely that the more effective electron transport took place for the 25:75 film-based solar cells with low-crystallinity ETLs. In this regard, Dorman et al. reported the increased charge mobility of high-temperature processed Sn-doped TiO 2 , whereas charge carrier recombination can be decreased in solar cells [11]. Since the ionic radius of Sn(IV) is 69 p.m., which is larger than Ti(IV) (61 p.m.) [17], it is thought that Sn-doped TiO 2 lowered the crystallinity, and the ETL had positive effects on the improved PCEs.  Figure 9 presents the distributions of I-V parameters obtained from each solar cell consisting of the 10 different ETLs. The data clearly show that there is a correlation between Jsc and PCE. In other words, the enhanced Jsc led to a marked improvement in PCEs. Considering that the samples (Sn/Ti = 100:0, 75:25, 50:50) had higher crystallinity compared to those of Sn/Ti = 25:75, it is most likely that the more effective electron transport took place for the 25:75 film-based solar cells with low-crystallinity ETLs. In this regard, Dorman et al. reported the increased charge mobility of high-temperature processed Sn-doped TiO2, whereas charge carrier recombination can be decreased in solar cells [11]. Since the ionic radius of Sn(IV) is 69 p.m., which is larger than Ti(IV) (61 p.m.) [17], it is thought that Sn-doped TiO2 lowered the crystallinity, and the ETL had positive effects on the improved PCEs.     As shown in Figure 10a, the lead halide perovskite had a grain boundary even for the best-performing device. The approximate thickness of the ETL was 100 nm (Figure 10b). The cross-sectional SEM image in Figure 10b enabled us to observe the smooth surface of the thin film. In addition, the transmittance spectrum of the Sn/Ti oxide film in Figure 10c showed that the incident light can ensure efficient absorption in the perovskite layer in the device. The optimization of the deposition condition of the perovskite precursor solution and ETL thickness will further improve efficiencies by the smaller grain boundary formation of perovskite and the efficient electron generation and transport at the material interface. Most notably, a simple one-time deposition process of the Sn/Ti ETL using the mixed ethanol/terpineol/PEG base liquid made it possible to realize the respectable PCE. As shown in Figure 10a, the lead halide perovskite had a grain boundary even for the best-performing device. The approximate thickness of the ETL was 100 nm (Figure 10b). The cross-sectional SEM image in Figure 10b enabled us to observe the smooth surface of the thin film. In addition, the transmittance spectrum of the Sn/Ti oxide film in Figure 10c showed that the incident light can ensure efficient absorption in the perovskite layer in the device. The optimization of the deposition condition of the perovskite precursor solution and ETL thickness will further improve efficiencies by the smaller grain boundary formation of perovskite and the efficient electron generation and transport at the material interface. Most notably, a simple one-time deposition process of the Sn/Ti ETL using the mixed ethanol/terpineol/PEG base liquid made it possible to realize the respectable PCE. As shown in Figure 10a, the lead halide perovskite had a grain boundary even for the best-performing device. The approximate thickness of the ETL was 100 nm (Figure 10b). The cross-sectional SEM image in Figure 10b enabled us to observe the smooth surface of the thin film. In addition, the transmittance spectrum of the Sn/Ti oxide film in Figure 10c showed that the incident light can ensure efficient absorption in the perovskite layer in the device. The optimization of the deposition condition of the perovskite precursor solution and ETL thickness will further improve efficiencies by the smaller grain boundary formation of perovskite and the efficient electron generation and transport at the material interface. Most notably, a simple one-time deposition process of the Sn/Ti ETL using the mixed ethanol/terpineol/PEG base liquid made it possible to realize the respectable PCE.

Conclusions
Nanoparticle-based uniform Sn/Ti metal oxide films were successfully obtained via sintering through optimization of the composition of precursor solutions containing titanium and tin salts, which were dissolved in an ethanol/terpineol/PEG base liquid. It was suggested that PEG also had a key role in improving the homogeneity of the film during sintering.
We demonstrated that the maximum power conversion efficiency was obtained for the concentration condition of [SnCl2·2H2O]/[titanium tetraisopropoxide (TTIP)] = 25:75 in the terpineol/PEG-based precursor solution. The combination of XRD and high-resolution TEM analysis was a straightforward approach to understanding the microstructures related to the crystallinity and fusion of nanoparticles depending on the sintering strategies. It appears that the lower crystallinity for the 25:75 Sn/Ti oxide films has significant potential that boosts the power conversion efficiency of the device. Such fundamental information on semiconductor Sn/Ti oxide hybrid layers will be beneficial for the further development of cost-effective high-performance perovskite solar cells [18].

Conclusions
Nanoparticle-based uniform Sn/Ti metal oxide films were successfully obtained via sintering through optimization of the composition of precursor solutions containing titanium and tin salts, which were dissolved in an ethanol/terpineol/PEG base liquid. It was suggested that PEG also had a key role in improving the homogeneity of the film during sintering.
We demonstrated that the maximum power conversion efficiency was obtained for the concentration condition of [SnCl 2 ·2H 2 O]/[titanium tetraisopropoxide (TTIP)] = 25:75 in the terpineol/PEG-based precursor solution. The combination of XRD and high-resolution TEM analysis was a straightforward approach to understanding the microstructures related to the crystallinity and fusion of nanoparticles depending on the sintering strategies. It appears that the lower crystallinity for the 25:75 Sn/Ti oxide films has significant potential that boosts the power conversion efficiency of the device. Such fundamental information on semiconductor Sn/Ti oxide hybrid layers will be beneficial for the further development of cost-effective high-performance perovskite solar cells [18].